Connecting rods for automobile engines have a large end with a separable cap for attachment to a crankshaft, and a small end typically with a cylindrical ring bearing assembly for connection to the piston utilizing a pin.
With the development of more powerful engines operating at higher rpm's, the connecting rods have been subjected to increased stress, calling for improvements in design.
Connecting rods were originally made by casting or forging separate attachable cap and body portions. The parts were separately machined at both joining and thrust faces, and then separately drilled with holes to accept fasteners.
An early step in the improvement of connecting rods was to cast the rod as a single piece, followed by drilling for fasteners. The single piece was then sawed to obtain cap and body portions which were separately rough-machined at the thrust and contacting surfaces. The two portions were then bolted together for finish-machining. Not only were these separate steps cumbersome and expensive, but also they did not ensure a perfectly matched cap and body under all conditions. In some cases, inherent diametrical fastener clearances permitted shifting between the cap and the body portion, which in turn shortened the bearing life.
The next step in this development was to forge a single-piece connecting rod which was subsequently split or cracked into a cap portion and a body portion, the intention being to provide non-sliding surfaces where the cap and the body portions are bolted together. The intention was that the surfaces would be properly remated, with the roughness of the cracked surface preventing any microshifting and thus assuring accurate operational alignment. To split the single piece into two portions, it was initially struck on one side with a sharp blow. However, this was unsuccessful because it was impossible to control the cracking plane and to prevent possible damage to the connecting rod.
An early attempt to solve this problem involved the insertion of a wedge-expandable mandrel into the large bore of the rod, as set forth in U.S. Pat. No. 2,553,935, issued to Parks et al in May, 1951. The idea was that the big end of the rod would fracture at the two weak sides of the yoke. The cracking was carried out at normal temperatures even though the rod was made of a strong, non-brittle, high carbon rough steel. Radial reductions at the intended cracking plane were provided by sawing, milling, drilling or a combination of these three. This reduced the crackable section and weakened the material to assist cracking. However, this approach did not ensure distortion-free cracked surfaces.
Another approach is set forth in U.S. Pat. No. 3,751,080 issued to John M. Bailey et al on Aug. 7, 1973. This patent recognizes the difficulty of fracturing strong high-carbon steels at room temperature when they were formed in large sizes. According to the patent, an electron beam is moved along a desired path in an undulating fashion which separates the rod to render a pair of rippled interfacing surfaces. It was found, however, that the electron beam imparted a deleterious effect to the material, in addition to being slow and costly.
Another approach is provided in U.S. Pat. No. 3,994,054, issued to Angus N. Cuddon-Fletcher et al on Nov. 30, 1976, in which tension forces are provided mechanically by conical pins forced into bolt holes at each side of the big end of the connecting rod. The bolt holes reduce the split plane section and the tapered pins provided a more equalized cracking impact. The technique, however, resulted in wear on the sides of the bolt openings and inhibited accurate remating.
Further approaches are provided in U.S. Pat. No. 4,569,109, issued to Mohamed A. Fetouh on Feb. 11, 1986, and U.S. Pat. No. 4,768,694, issued to Alroy G. Fabris on Sep. 6, 1988. This patents are directed to applying freezing or heat treatment to a rod composed of either cast iron, aluminum or steel. High impact tension forces are applied across a cracking plane defined by two notches in the internal surface of the large end bore while limiting relative movement to avoid ductile bending or incomplete fracture. It was found that embrittlement by freezing or heat treatment led to an indefiniteness in the direction of the crack, and as much as 25% of a production run had to be scrapped because of an improper placement of the final crack planes.
U.S. Pat. No. 4,970,783, issued to Olaniran et al on Nov. 20, 1990, is directed to a method of making a connecting rod which involves treating the intended cracking locations with hydrogen to facilitate hydrogen stress cracking.
A patent of more general interest is U.S. Pat. No. 4,993,134, issued to Hoag et al on Feb. 19, 1991.